On the nature of ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) that are located in external galaxies exhibit X-ray luminosities exceeding those of the brightest black holes in the Milky Way and the Local Group galaxies by hundreds or even thousands of times. New classes of objects have been discovered. ultraluminous X-ray pulsars (ULXPs) and high-velocity outflows whose X-ray-range speed is up to 0.2c. The ULXs and ULXPs fully correspond to concepts of super-Eddington accretion. Five ULXs exhibit quasiperiodic oscillations and a flat-topped noise in the X-ray range power spectrum. Optical spectra of ULXs are very similar to those of SS433, late nitrogen stars (WNL/WR), or LBV (luminous blue variable) stars. The results obtained suggest that ULXs are systems that contain supercritical accretion disks.